Previous testing of argon-ion laser tubes made entirely of alumina showed ceramic failure or fracturing at levels of discharge current below those for which entirely satisfactory laser products can be made. The fracturing of a cylindrical, ceramic tube which has a source of heat on its inside diameter and a cooling sink on its outside diameter is caused by differential stress which develops across the cross section. The cooling sink can be established, for example, by a water jacket or air-cooled fins. It is known that the magnitude of the stress is proportional to the thermal conductivity, the heat flux density, and the dimensions of the part. When the stress exceeds the strength of the ceramic material, fracturing failure occurs. For a specific material, such as alumina, and a specific application, factors such as thermal conductivity, yield stress, and heat flux density are defined. Larger diameters and thinner wall thicknesses are known to minimize stress for a given flux density. However, other heat transfer and mechanical considerations are introduced.
An alternative to use of alumina is to use beryllia which has much better thermal conductivity and can be operated to a much higher power level without fracture. Beryllia, however, is far more expensive than alumina and is also toxic. While the toxicity problem can be circumvented by proper design and processing, the disadvantage of costs has not been overcome and the possibility of doing so is slight. There has, therefore, been significant effort to use alumina in conjunction with metals to improve the overall thermal capacity of an alumina system. This effort, however, leads to still further complex considerations of thermal impedance, mechanical rigidity and strength, wear performance, sealing, electrolysis, ionization in the gas return paths, longitudinal expansion, ease of production and quality control.
As further background, it has also been known to construct laser tubes as a continuous thick wall tube having a central discharge bore and gas return bores spaced radially outward from the discharge bore. Another form of construction has been to construct the laser tube from relatively thick wall segments with mating faces and aligned bores which provide the discharge and gas return bores. U.S. Pat. No. 3,670,262 illustrates such a segment-type construction. However, it has not been known to construct the laser tube from thin wall segments interfaced together, with the gas return bores formed in the thin walls of the segments and the gas discharge bore defined by apertures in refractory metal discs supported in the segments.
As a further aspect of the prior art, it has also been known to provide a continuous tube with a series of refractory metal discs centrally mounted on longitudinally-spaced annular metal supports. The main discharge bore was defined by holes in the center of the refractory metal discs and the gas return paths were defined by holes in the annular metal supports. However, such a single continuous tube construction has not proven to be practical in actual operation because of, among other reasons, not providing sufficient isolation between the main discharge bore and the gas return paths.
With the foregoing background and practical considerations in mind, the present invention is directed to providing a significantly improved segmented ceramic-metal laser tube construction. The improved tube construction is aimed at dealing with the mentioned operating and production considerations in a practical way and while primarily intended for use in water-jacketed, argon-ion lasers, the improved tube construction is also anticipated to find application in other types of laser tubes having fin-type cooling. The achieving of these and other objects will thus become apparent as the description proceeds.